I can't tell you everything he's doing. I'm not that skilled yet, but I can get us started on this one, and others can put in their two cents if you like.
Start with a very good high range. Do that all on Little Kid Voice, very highly placed. Shed all the weight. Add mask and stir with a dash of fast vibrato, used very sparingly. Allow your voice to break deliberately here and there for effect. Do a few licks and runs at the speed of the fast vibrato. Use a high-range mix of chest and head voice with good cord closure. Add some breathy head voice (falsetto) a couple of times. Keep it thin.
Put this in an oven, preheated to 98.6+ degrees, and bake until crisp and just the right blend of energy and emotion. Season to taste.
First you master your chest voice and stretch it, over time, to as high as you can get it where it overlaps head voice by as many notes as you possibly can.
Then you master your head voice, stretching it, over time, to as high as you can possibly get it and also stretching your head voice as Low as you possibly can to overlap your Mid/Call voice and your chest voice by as many notes as you possibly can.
Then you practice your mid voice, using mixtures of both head and chest qualities, to suit the song and your voice, as you choose. Mixed voice is literally a mixture of the various coordinations you have already mastered.
It feels very unstable at first, because you are used to using one register at a time or the other. Now you have to do a sort of balancing act that can feel a little out-of-balance. You get used to that feeling, like an astronaut has to get used to the feeling of falling all the time when they go into zero gravity. Eventually they don't notice that it feels off-balance anymore. Your voice will adapt similarly.
What is he trying to do? Sound sexy, sound free, hit lots of highs, and have a good time!